Richard Jinman

The mood in the room on the third floor of one of London's hippest hotels is decidedly unruly.

Saoirse​ Ronan​ has kicked off her shoes and is cackling with laughter from the depths of an oversized armchair as Greta Gerwig​ attempts to eat a plate of room service prawns without spitting them across the room.

Trailer: The Wound

Trailer: Lady Bird

In the early 2000s, an artistically-inclined seventeen-year-old comes of age in Sacramento, California.

For now at least the task of promoting their acclaimed new movie Lady Bird, a pitch perfect coming-of-age drama written and directed by Gerwig and starring Ronan as a high school student who dreams of escaping her dreary Californian city, has been put on hold.

Talk turns to the most desirable colour for an iPhone. "I wanted a pink one," says Ronan. "But they didn't have pink in the shop."

Directors often grow close to the actors they work with and if they don't they tend to keep quiet about it. But the rapport between Gerwig and the prodigious 23-year-old star of Atonement, The Lovely Bones and Brooklyn, goes way beyond mutual admiration. They act like best mates who have known each other for years, poking fun at each other and finishing each other's sentences.

The riotous mood may have something to do with the buzz their film is generating as, one by one, the world's movie critics succumb to its charms. "I wish I could convey to you just how thrilling this movie is," wrote A. O Scott in The New York Times; "It's unique and original and fresh and wonderful, and can you tell I loved it?" gushed Richard Roeper in the Chicago Sun-Times. A few weeks before Christmas this modest $US10 million drama eclipsed Toy Story 2 to become the film with the longest unbroken streak of positive reviews – 195 and counting – on the Rotten Tomatoes website.

Saoirse Ronan, left, and Laurie Metcalf in a scene from Lady Bird. Metcalf has been nominated for an Oscar for best supporting actress. Photo: AP

Now, the awards – and Oscar nominations – have followed. First came the New York Film Critics Circle and the Gotham Independent Film awards for Ronan, who was last year named best actress. In January, the National Society of Film Critics and the Golden Globes followed suit, with the critics awarding it best picture, best director and best supporting actress for Laurie Metcalf​.

The next day the Golden Globes added to the hype, with Ronan named best actress in a comedy, while the film was declared best comedy. Then, nearly two weeks later, the Academy Awards came knocking, with five nominations for best picture, best director, best actress, best supporting actress and best original screenplay.

For Gerwig, the nomination sees her joining an elite club: she is now one of only five women the Academy has nominated for best director since the awards began in 1929. Ronan, meanwhile, has already been nominated twice: best supporting actress for Atonement in 2008 and best actress for Brooklyn in 2016.

Lady Bird's success seems all the more remarkable because it is Gerwig's first solo outing as a director (she co-directed the 2008 drama Nights and Weekends with Joe Swanberg​). The 34-year-old has more than paid her dues, however. Both as a writer whose collaborations with the writer and director Noah Baumbach​ – he is also her partner – include Frances Ha and Mistress America and as an actor whose name is synonymous with smart, sharply observed dramas about women such as Maggie's Plan and 20th Century Women. Who better to turn a script about a free spirit called Christine who demands to be known as Lady Bird – the nickname of Claudia Johnson, America's most beloved First Lady – into an all-conquering film?

Not that it was easy. "The thing about directing a movie is that the entire time you're scanning for what's wrong with it, what's not working, what's not good enough," she says. "It makes you relentless. You get so used to looking for the flaws it's hard to let go. I was at the New York Film Festival and Noah said, 'Are you still taking notes?'. And I said: 'I can't stop!'."

It is tempting to see Lady Bird as an autobiographical film because Gerwig grew up in Sacramento, the Californian city where it is set and like her protagonist she attended a Catholic high school. Yes, it's "deeply personal", she says. But the character of Lady Bird is not based on her teenage self.

Gerwig insists she was a conformist, a rule follower, whereas Lady Bird often seems at war with her family and her surroundings. She has incendiary clashes with her mother (played with brilliant sensitivity by Metcalf), steals her teacher's grade book and scoffs packets of pilfered communion wafers with her best friend Julie (Beanie Feldstein​). She falls in and out of love, has awkward fumbling sex and dreams of escaping to the East Coast, a place she imagines is full of "culture" and "writers who live in the woods".

In hindsight, the choice of Ronan for the film's lead role seems inspired. But while she was born in New York and is usually described as an Irish American actor, she had no direct experience of life in a US high school having moved back to Ireland at the age of three. Her name – it's pronounced Ser-sha – and her lilting accent are both distinctly Irish.

None of that mattered when Gerwig met Ronan for the first time in a hotel room at the 2015 Toronto Film Festival and they read the script together – Ronan playing Lady Bird; Gerwig playing everyone else. "I knew within the first two pages that she was Lady Bird and that she had the part," the director says. "But I didn't stop her because I was being selfish and wanted to hear it all read out."

In Ronan she found an actor who could interpret a style of writing that sidesteps big dramatic moments in search of more subtle truths. "So much of my writing is in the cracks," explains Gerwig. "I'm always concerned because some of it seems so subtle and delicate. I worry if people are going to get it or if it just exists in my mind. So when an actor does something that just clarifies it all you go, 'Thank god for you'."

Ronan inhabits the role of a rebellious adolescent to perfection. From her messy home-dyed red hair and thrift store clothes to her acerbic humour and tempestuous mood swings, Lady Bird is an unforgettable creation. Even the character's mild acne – which I had assumed was the work of a skilled make-up artist – turns out to be real. Ronan's skin wasn't in great shape when filming began, but rather than cover it up she decided it was an opportunity to "let a teenager's face actually look like a teenager's face in real life".

She looks very different today, of course. Her skin is positively glowing and her blonde shoulder-length hair is shampoo ad glossy. Leather trousers and a chic white sweater complete the movie star look. The British tabloids claim she has been dating the Irish musician Hozier since 2016 when she appeared in a video for his song Cherry Wine. But she has never confirmed the rumours and is disinclined to discuss her private life.

I ask her if it was more difficult working with a director who had not only conceived and written her role, but is an acclaimed actor in her own right?

Before she can answer, Gerwig interjects. "It didn't help that I would put on her costume at night." They both crack up laughing.

"It's a fair point," says Ronan when she regains her composure. "If you're working with a director who has never acted you know that you bring one thing to the table and they bring something different.

"But the fact that Greta is an actor as well as a director means she has sensitivity towards actors that others might not. She knows how we think and if I was ever insecure about something she knew why I would be insecure and what she would need to say to make me feel better and able to do the scene."

Both Gerwig and Ronan are uncomfortable taking credit for the film's success, but they clearly know they've made something remarkable. Ronan, who has seen the movie twice – once with her mother and once with her best friend Eileen – says she usually dreads seeing herself on the big screen. Lady Bird was different. "I have never had the experience before where I've watched something I'm in and within a few seconds I'm like, 'This is great isn't it?'," she says. "That has never happened before. It was extraordinary."